Barriers to appropriate HIV treatment and care for HIV-infected injection drug users (IDU) are a growing concern. International efforts to improve the delivery of highly active antiretroviral therapy (HAART) to IDU, such as the WHO'S 3 by 5 Initiative, are in urgent need of data on how these barriers can be addressed. While there has been much focus on HIV prevention for HIV-negative IDU, there has recently been a growing recognition that breaking the cycle of HIV transmission will require renewed focus on behaviors and prevention needs of HIV-infected IDU. This proposal seeks to fill the information gap existing in North America and elsewhere through the following specific aims: 1. To evaluate the incidence of HIV/AIDS service uptake (e.g., antiretroviral therapy) and factors associated with uptake among IDU 2. To evaluate the incidence of HIV antiretroviral drug resistance among HIV-infected IDU and the role of unique antiretroviral delivery models on the time to antiretroviral resistance 3. To evaluate trends in HIV disease progression and factors such as medical service and secondary prevention service use that may be associated with reduced disease progression among IDU 4. To determine factors associated with persistent HIV risk behaviors (e.g., syringe lending) in a setting with comprehensive evidence-based HIV prevention strategies for HIV-infected IDU. These aims will be met through the development of a new prospective cohort study of HIV-infected IDU in Vancouver, Canada. This setting is unique, since all medical care and antiretrovirals are provided free of charge through a universal healthcare system, allowing the research program to focus on non-financial barriers to care. The cohort will involve serial blood sampling to evaluate antiretroviral resistance patterns, a detailed questionnaire to evaluate illicit drug use and related behaviors, and ongoing administrative linkages to the province's centralized medical care and social service databases to evaluate access to antiretrovirals and access to other HIV/AIDS care and prevention services. By helping to fill a critical information gap, this study has the potential to make a major contribution to public health policies for IDU.